The present invention relates generally to electronic ballasts for powering a discharge lamp. More particularly, the present invention relates to a discharge lamp ballast using an auto-transformer for high voltage detection in a resonant circuit.
Discharge lamp ballasts (or more specifically a high-pressure discharge lamp ballast) are conventionally known in the art for lighting high-pressure discharge lamps such as metal halide and high-pressure mercury lamps (high-intensity discharge lamps, which are also referred to as HID lamps).
An example of such a discharge lamp ballast is shown in FIG. 9 and includes a power supply circuit 100 for supplying a rectangular wave voltage with a polarity inverted at a predetermined frequency to a discharge lamp La (applying the voltage between both electrodes of the discharge lamp La), a resonant circuit 200 for supplying an output voltage corresponding to the frequency of the rectangular wave voltage of the power supply circuit 100 to the discharge lamp La (applying the voltage between both electrodes of the discharge lamp La), a voltage detection circuit 300 for detecting the output voltage of the resonant circuit 200 and a control circuit 400.
The power supply circuit 100 includes a power factor correction (PFC) circuit 110, a step-down converter circuit 111 and an inverter circuit 112.
The PFC circuit 110 outputs a DC voltage based on electric power obtained from an AC power source AC, and includes a filter 110a, a rectifier 110b and a step-up converter circuit 110c. The filter 110a further includes two capacitors C100, C101 and a choke coil (common mode choke coil) LF. The step-up converter circuit 110c raises an output voltage of the rectifier 110b and includes an inductor L100, a diode D100, a switch Q100, a resistor R100, capacitors C102, C103 and so on. The switch Q100 of the step-up converter circuit 110c is controlled by a step-up controller 113.
The step-down converter circuit 111 lowers an output voltage of the step-up converter circuit 110c and includes a switch Q101, a diode D101, an inductor L101 and so on. The switch Q101 of the step-down converter circuit 111 is controlled by a step-down controller 114. The power supply circuit 100 also includes a resistor R101 for detecting a lamp current of the discharge lamp La, a capacitor C104 for smoothing an output voltage of the step-down converter circuit 111, and resistors R102, R103 for detecting the output voltage of the step-down converter circuit 111.
The inverter circuit 112 supplies the rectangular wave voltage at a predetermined frequency to the discharge lamp La (applying the voltage between the both electrodes of the discharge lamp La). The inverter circuit 112 is a full-bridge circuit formed of four switches Q102 to Q105. In the inverter circuit 112, a connection point of the switch Q102 and the switch Q103 is a first output end and a connection point of the switch Q104 and the switch Q105 is a second output end. The discharge lamp La is coupled between the first and second output ends.
The resonant circuit 200 includes a resonant inductor L200 formed of a coil and a resonant capacitor C200. The resonant inductor L200 is inserted between one output end of the inverter circuit 112 and one electrode of the discharge lamp La. The resonant capacitor C200 is connected in parallel with the discharge lamp La. The resonant frequency of the resonant circuit 200 is determined depending on an inductance of the resonant inductor L200 and a capacitance of the resonant capacitor C200. The output voltage of the resonant circuit 200 is equal to the resonant voltage and is determined by the resonant frequency, and a frequency and amplitude of the rectangular wave voltage of the power supply circuit 10.
The voltage detection circuit 300 includes capacitors C300 to C304, resistors R301 to R304 and diodes D300, D301. The voltage detection circuit 300 detects a potential of a connection point between the resonant inductor L200 and the resonant capacitor C200.
The control circuit 400 controls turning ON/OFF of the switches Q102 to Q105 of the inverter circuit 112.
To light the discharge lamp La, it is necessary to supply a resonance voltage which is higher than a starting voltage to the discharge lamp La, thereby causing dielectric breakdown.
For this reason, the control circuit 400 has a starting mode for starting the discharge lamp La as an operation mode. In the starting mode, the control circuit 400 sets the frequency of the rectangular wave voltage of the power supply circuit 100 based on the detection result of the voltage detection circuit 300 so that the output voltage of the resonant circuit 200 exceeds the starting voltage of the discharge lamp La.
The starting voltage of the above-mentioned high-pressure discharge lamp is generally very high, and for example, in the starting mode, there are cases where the voltage as high as 3000 V must be supplied. For this reason, it is necessary to design the voltage detection circuit 300 so as to have a high voltage resistance, that is, to be able to withstand a high voltage. Specifically, the number of circuit components of the voltage detection circuit 300 is increased to lower the voltage exerted on individual components or high voltage resistance components are used as circuit components for the voltage detection circuit 300.
When the number of components is increased as in the former case, as a matter of course, the scale of the voltage detection circuit 300 becomes large. When high voltage resistance components are used as in the latter case, because these components are generally larger than low voltage resistance components, the scale of the voltage detection circuit 300 also becomes large.
For this reason, in the conventional discharge lamp ballast, the size of the voltage detection circuit 300 becomes large to account for the high startup voltage, resulting in that the discharge lamp ballast is disadvantageously grown in size.
When the resonant circuit 200 includes only the single resonant inductor L200 as shown in FIG. 9, the voltage which can be output from the resonant circuit 200 is determined depending on the output voltage of the step-down converter circuit 111. For this reason, to supply an appropriate voltage to the discharge lamp La, the output voltage of the step-down converter circuit 111 needs to be increased. However, when the output voltage of the step-down converter circuit 111 is increased, it is necessary to use high voltage resistance switching elements as the switches Q102 to Q105 of the inverter circuit 112, disadvantageously leading to a further increase in losses.